1. Field of the Invention
The present invention relates to the field of flow batteries and, in particular, to methods and systems for collecting current in a flow battery.
2. Discussion of Related Art
Flow batteries store electrical energy in a chemical form, and subsequently dispense the stored energy in an electrical form via a spontaneous reverse reduction-oxidation (redox) reaction.
As such, a flow battery is an electrochemical storage device in which an electrolyte containing one or more dissolved electro-active species flows through a reactor cell where chemical energy is converted to electrical energy. Conversely, the discharged electrolyte can be flowed through a reactor cell and electrical energy converted to chemical energy. Electrolyte is stored externally, for example in tanks, and flowed through a set of cells where the electrochemical reaction takes place. Externally stored electrolytes can be flowed through the battery system by pumping, gravity feed, or by any other method of moving fluid through the system. The reaction in a flow battery is reversible. The electrolyte, then, can be recharged without replacing the electroactive material.
The minimal unit that performs the electrochemical energy conversion is generally called a “cell”, whether in the case of flow batteries, fuel cells or secondary batteries. A device that integrates many such cells, coupled electrically in series or parallel, to get higher current or voltage or both, is generally called a “battery”. However, it is common to refer to any collection of coupled cells, including a single cell used on its own, as a battery. As such, a single cell can be referred to interchangeably as a “cell” or a “battery”.
Flow batteries can be utilized in many technologies that require the storage of electrical energy. For example, flow batteries can be utilized for storage of night-time electricity that is inexpensive to produce to provide electricity during peak demand when electricity is more expensive to produce or demand is beyond the capability of current production. Such batteries can also be utilized for storage of green energy (i.e., energy generated from renewable sources such as wind, solar, wave, or other non-conventional sources).
Many devices that operate on electricity are adversely affected by the sudden removal of their power supply. Flow batteries can be utilized as uninterruptible power supplies in place of more expensive backup generators. Efficient methods of power storage can provide for devices to have a built-in backup that mitigates the effects of power cuts or sudden power failures. Power storage devices can also reduce the impact of a failure in a generating station. Other situations where uninterruptible power supplies can be of importance include, but are not limited to, buildings where uninterrupted power is critical such as hospitals. Such batteries can also be utilized for providing an uninterruptible power supply in developing countries, many of which do not have reliable electrical power sources resulting in intermittent power availability.
Another possible use for flow batteries is in electric vehicles. Electric vehicles can be rapidly “recharged” by replacing the electrolyte. The electrolyte can be recharged separately and reused.
The flow cell works by changing the oxidation state of its constituents during charging or discharging. The basic flow cell includes two half cells connected in series by the conductive electrolyte, one for anodic reaction and the other for cathodic reaction. Each half cell includes an electrode with a defined surface area upon which the redox reaction takes place. Electrolyte flows through the half cell as the redox reaction takes place. The two half cells are separated by an ion-exchange membrane (IEM) where either positive ions or negative ions pass through the membrane. Multiple such cells can be electrically coupled (e.g., stacked) either in series to achieve higher voltage or in parallel in order to achieve higher current. The reactants are stored in separate tanks and dispensed into the cells as necessary in a controlled manner to supply electrical power to a load.
Problems that can affect the performance of a flow cell are the transfer of current from the site where the electrochemical reaction takes place and the collection of current at an electrode of the flow cell. Various factors such as defects on the surface of electrodes, material composition and properties of the electrodes, sealing of the electrodes, and other such factors affect the flow and collection of current. There is, therefore, a need for improved current collection in order to increase the performance and efficiency of a flow battery.